Like the article Nancy linked, this one is pretty heavily weighted toward the GPO since that's what most public aquariums feature. As Nancy pointed out though, most of the ideas could be scaled down.

OTOH: I suspect that the enrichment issue would be moot in an aquarium designed to simulate the octo's natural environment. It goes without saying that most species of octopus need dens to feel safe but it's rare for anyone to discuss the advantages of providing a variety of live food AND providing the food animals with places to hide or escape. Not only would that provide enrichment (hunting) for the octo but it would also reduce the 'pollution' problems associated with rotting food; until the octo inevitably catches a given prey item it's just another part of the display. Further, in the case of many small crabs and shrimp, these prey items will provide a certain amount of janitorial service for the tank.